The utilization of compressed video utilizing standards such as MPEG-2 have enabled video to penetrate consumer markets that would otherwise be impossible. Compressed video can now be encoded onto a CD-ROM or be transmitted via satellite communications using small 18-inch satellite dishes, for example. The use of motion compensated compressed video such as found in MPEG-2requires the storage of reference frames for the decoding of the motion compensation encoded frames. For example, in MPEG-2encoded video, three types of pictures are utilized. Intrapictures (I Pictures) provide access points for random access but only with moderate compression. Predicted pictures (P Pictures) are coded with reference to a past picture which can be an I Picture or a P Picture and in general are used as a reference for future predicted pictures. Bidirectionally predicted pictures (B Pictures) provide the highest amount of compression but require both a past and a future reference for prediction. These pictures can never be used as a reference. In a series of pictures which are to be displayed in the following order:
IB.sub.1 B.sub.2 P.sub.1 B.sub.3 B.sub.4 P.sub.2 PA1 IP.sub.1 B.sub.1 B.sub.2 P.sub.2 B.sub.3 B.sub.4.
where I represents an I Picture, P represents a P Picture and B represents a B Picture, he pictures would have to be decoded in the following order:
This requires that I and P Pictures be stored in order that they can be utilized for decoding B Pictures. If the B Pictures can be decoded and displayed on the fly, then it is not necessary to store these pictures. If they cannot, B Pictures must also be stored. If the pictures are sent on a frame basis rather than a field basis, then decoding of the B picture on the fly will still require the storing of at least the not yet displayed field, and more commonly a portion of the displayed field as well. This information is typically stored in a RAM. As is well known, RAM memories are available in discrete sizes. If a conventional memory allocation technique is utilized, the size of the RAM memory that is required will exceed one size of memory by a small amount, thus requiring a system which utilizes approximately twice the necessary memory. In a consumer product, the cost of this additional memory unnecessarily increases the cost of the consumer device thereby making such products non-competitive in the marketplace and/or increasing the price of the device beyond what consumers are willing to pay.
Accordingly, there is a need for a technique to reduce the amount of memory needed to store the information necessary to decode such encoded pictures.